Restrict the version constraints on exported dependencies.

pkg/polymer/pubspec.yaml

 name: polymer
-version: 0.15.2
+version: 0.15.3
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 description: >
   Polymer.dart is a new type of library for the web, built on top of Web
   Components, and designed to leverage the evolving web platform on modern
   browsers.
 homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
   analyzer: '>=0.15.6 <0.23.0'
   args: '>=0.11.0 <0.13.0'
   barback: '>=0.14.2 <0.16.0'
   browser: '>=0.10.0 <0.11.0'
   code_transformers: '>=0.2.3 <0.3.0'
   html5lib: '>=0.12.0 <0.13.0'
   logging: '>=0.9.2 <0.10.0'
-  observe: '>=0.11.0 <0.13.0'
   path: '>=0.9.0 <2.0.0'
   polymer_expressions: '>=0.12.0 <0.14.0'
   smoke: '>=0.2.0 <0.3.0'
   source_maps: '>=0.9.4 <0.11.0'
   source_span: '>=1.0.0 <2.0.0'
   template_binding: '>=0.12.0 <0.15.0'
   web_components: '>=0.10.0 <0.11.0'
   yaml: '>=0.9.0 <3.0.0'
+
+  # Because polymer exports observe, it needs to keep its version constraint
+  # tight to ensure that a constraint on polymer properly constraints all
+  # features it provides.
+  observe: '>=0.12.2 <0.12.3'

[Jennifer Messerly, 2014/12/04 23:05:20]

Nice solution. Thanks so much for tracking this down!
BTW, we could just remove this re-export from Polymer. It's not worth it, IMO.
In fact, new Polymer 0.8 doesn't depend on observe-js so the timing is perfect.
But naturally feel free to defer that to Jake or Siggi, since it would be a
major version bump. Up to you :)

 dev_dependencies:
   unittest: '>=0.10.0 <0.12.0'
   markdown: '>=0.7.0 <0.8.0'
 transformers:
 - polymer/src/build/mirrors_remover:
     $include: lib/polymer.dart
 - code_transformers/src/delete_file:
     $include:
       - lib/src/build/log_injector.css
       - lib/src/js/polymer/polymer.js
 - observe:
     files: lib/src/instance.dart
     $include: lib/src/instance.dart
 environment:
   sdk: '>=1.4.0 <2.0.0'